"I don't think there is any doubt left for anyone who actually looks at the science," Clinton said. "There are still some holdouts, but they are fighting a losing battle. The science is overwhelming, but what is deeply concerning is that climate change is accelerating."

Graham, who declared himself "on the fence" about climate change legislation, said an academic debate about global warming is different in the North.

"If you can go to the Native people and listen to their stories and walk away with any doubt that something's going on, I just think you're not listening," he said.

McCain and Sen. Joe Lieberman, D-Conn., are sponsoring legislation that would limit greenhouse gas emissions from utilities and industry. The Climate Stewardship and Innovation Act would cap U.S. emission levels at levels recorded in 2000.

McCain said the trip has been valuable for the accumulation of evidence that can be used to push the bill. Ultimately, he said, Americans will demand laws to decrease emissions, just as they demanded campaign financing reform.

"It's coming up from the bottom," he said. "It's the special interests versus the people's interests and I still have enough confidence in our system of government that the people's interest will ultimately prevail."

Collins said the senators were approached by Alaska guides who thanked them for taking time to look at how climate change affects Alaska. They echoed what indigenous people in Canada told the senators.

"I don't think anyone who has talked to these individuals as well as the scientists would have any doubt that this is a real and growing problem," she said.

McCain said his bill continues to face opposition from industry, but that may change from businesses that operate overseas.

"They have to do business in Europe, and thereby comply with the requirements for the reduction of greenhouse gas emissions," he said. "You will see more and more international corporations going in that direction because they have to."

Graham couched the argument for climate change, as well as another major Alaska issue, petroleum drilling of the Arctic National Wildlife Refuge, as a national security measure. Continued dependence on foreign fossil fuels makes America vulnerable, he said.

"The sooner we get started with alternative energy sources and recognize that fossil fuels makes us less secure as a nation, and more dangerous as a planet, the better off we'll be," Graham said.

Opponents who ignore evidence of humans contributing to climate change, Clinton said, are participating in a trend of turning Washington, D.C. into what she calls an "evidence-free zone."

"You just keep saying something no matter how untrue and unfactual it might be, over and over and over again, and try to drive the politics to meet your ideological or commercial agenda," she said. "That is a grave disservice to our country."